Will the plane fly?

[Scottwax]

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?



This question is originally from a physics board and there is not a firm consensus regarding the ability to the plane to take off.

[gb387]

I am no expert.... but here is my first thought, Seems to me the plane would be sitting still.... Wheels and conveyor would be moving but the the plane would be sitting there, right? So no sir over the wings = no lift. Or not?

For example... if you walk in the opposite direction of one of those moving sidewalks in the airport.... you can match the speed of the walkway, you are still walking but you are going no where.

scratch the above, I have re thought it

EDIT: now that I think about it a little more.... the plane may actually be be able to take off. Do the wheels have anything to do with it? The engine is pulling the plane through the air, no matter how fast the conveyor is moving the wheels would just continue to move faster(not the air speed, this the planes engine is going to control).

OK, I give I am getting a headache thinking about it. Sounds like a subject for Mythbusters!

[DFTowel]

I don't quite get why this is a difficult answer to determinne. The situation is this... the plane attempts to move forward but cannot due to the fact that the runway/conveyer is moving the opposite way at the same speed, therefore the plane is standing still and has an airspeed of zero. Of course the plane will not fly! To fly it's need air moving at a certain speed over and under the wings to create lift, therefore, if the plane is essentially standing still and has zero airspeed it cannot lift off.

[Skuz]

DFTowel explained my thought.

[Desertdawg]

Put a skateboard on a conveyor belt, and push it with a stick...

Does it matter how fast the conveyor is going?
All the matters is how hard you push on the stick (jet engine)...

The plane will fly because it is propelled by the jet engines, not the wheels...

I still want to know why we call them buildings???
If its finished, shouldn't we call them Builts !

[Jeff Smith]

If the conveyor is movng backwards and matching the airplanes foreward speed, The plane will not fly as the is no lift under the wings to make it fly.

It would be impossible. Would not matter if a jet or prop rotor aircraft.

So I agree with DF Towel here. An aircraft must have foreward motion to fly. Or be placed in a wind tunnel. There must be wind moving under and over the wings.

[Ranger72]

Its called Bernoulllis principal.... The wing is shaped in such a way that it forces the air to travel faster on top of the wing... This creates a drop in preasure giving the ability for lift.... So no, it won't fly.

[the other pc]

Lift is created by airflow over the wings.

If there is sufficient airflow over the wings the plane will lift off.

If we assume:

any effects of air dragged along the surface of the conveyor are negligible,

the conveyor is sufficiently (most texts would say to assume infinitely unless specified) long,

inertia of the landing gear is negligible,

friction in the landing gear is negligible,

forward acceleration of the airplane is exclusively derived from its flight propulsion system (jet or prop) thrust,

Then the result will be:

The airplane will accelerate forward normally (albeit with the “runway” traveling “backwards” relative to the airplane, doubling the “ground” speed of the landing gear, whose effect is negligible), attain takeoff airspeed and lift off.


If we assume:

inertia of the landing gear is substantial,

Then the result will be:

The airplane will accelerate forward but the extra energy required for angular acceleration of the landing gear will need to be compensated by propulsion thrust, lengthening the time and distance to achieve takeoff airspeed.


If we assume:

friction in the landing gear is substantial,

Then the result will be:

The airplane will accelerate forward but the extra thrust required to overcome friction of the landing gear will need to be provided by the propulsion system. If the system does not have sufficient thrust margin the airplane will not achieve takeoff airspeed. If the system does have sufficient thrust margin the airplane will achieve takeoff airspeed at a greater distance, after a longer time.


PC.

[Hannibal]

yes it will fly

[Jeff Smith]

Originally posted by the other pc
Lift is created by airflow over the wings.

If there is sufficient airflow over the wings the plane will lift off.

If we assume:

any effects of air dragged along the surface of the conveyor are negligible,

the conveyor is sufficiently (most texts would say to assume infinitely unless specified) long,

inertia of the landing gear is negligible,

friction in the landing gear is negligible,

forward acceleration of the airplane is exclusively derived from its flight propulsion system (jet or prop) thrust,

Then the result will be:

The airplane will accelerate forward normally (albeit with the “runway” traveling “backwards” relative to the airplane, doubling the “ground” speed of the landing gear, whose effect is negligible), attain takeoff airspeed and lift off.


If we assume:

inertia of the landing gear is substantial,

Then the result will be:

The airplane will accelerate forward but the extra energy required for angular acceleration of the landing gear will need to be compensated by propulsion thrust, lengthening the time and distance to achieve takeoff airspeed.


If we assume:

friction in the landing gear is substantial,

Then the result will be:

The airplane will accelerate forward but the extra thrust required to overcome friction of the landing gear will need to be provided by the propulsion system. If the system does not have sufficient thrust margin the airplane will not achieve takeoff airspeed. If the system does have sufficient thrust margin the airplane will achieve takeoff airspeed at a greater distance, after a longer time.


PC.

I agree with your thoughts except for that fact that he states that the conveyor has a control system which enables it to "match" the aircrafts foreward speed. Thus negating any foreward speed. Sticking to the facts as he has given, it would not matter what length the belt is as it only needs to be long enough for the aircraft to sit on as the aircraft is not moving. As the aircraft is not moving, there is no lift. Not disagreeing with you, just trying to stick to the facts as I perceive he has given.

Of course thing would only apply to a conventional aircraft. A STOVL (Short take-off & Vertical Landing) aircraft can compensate for the lack of forward airspeed simply by creating it's own lift. The JSF,Harrier and V-22 Osprey being examples of this. Not sure of any other examples of STOVL aircraft currently in product. Unless you take into consideration JATO assist, which can theoretically be used for any aircraft.

[Jeff Smith]

I think a neater question would be would an aircraft stay alot with a tailwind greater than the foreward speed needed. Or would it hover?

I'll ask one of our test pilots here in a couple of minutes.

[Don]

Ground speed is not relevant. The aircraft must still reach an airspeed (relative to its fusilage/wings) to generate the required lift to achieve take-off speed.

Initially, the aircraft is pushed along the ground until the speed of the air flowing over it's wings is enough to crate lift. If the aircraft does not achieve forward motion through the air, it will not take off (even assuming leanding gear and wheel to ground friction being 0)

In the given scenario, the ground speed, which is irrelevant, could be 1,000 miles an hour, but the aircraft still has 0 airspeed.

Yes, the propulsion of the aircraft is based on the engine, not the wheels, but it still must have forward motion in the air in order to achieve flight. Being on the treadmill forces the aircraft to remain stationary in the air, meaning 0 airspeed and 0 flight,

[Jeff Smith]

Originally posted by Don
Ground speed is not relevant. The aircraft must still reach an airspeed (relative to its fusilage/wings) to generate the required lift to achieve take-off speed.

Initially, the aircraft is pushed along the ground until the speed of the air flowing over it's wings is enough to crate lift. If the aircraft does not achieve forward motion through the air, it will not take off (even assuming leanding gear and wheel to ground friction being 0)

In the given scenario, the ground speed, which is irrelevant, could be 1,000 miles an hour, but the aircraft still has 0 airspeed.

Yes, the propulsion of the aircraft is based on the engine, not the wheels, but it still must have forward motion in the air in order to achieve flight. Being on the treadmill forces the aircraft to remain stationary in the air, meaning 0 airspeed and 0 flight,

I agree Don, If this were the not the case. Take away the conveyor belt. Lets say we have a parked aircraft, brakes locked, And let's say the brakes were strong enough to hold the bird. Is the aircraft just going to be able to pull enough power to up and fly away? No. The engine is not creating the lift. Only thrust. Gotta have that foreward motion/speed.

[Shiny Lil Detlr]

Originally posted by DFTowel
I don't quite get why this is a difficult answer to determinne. The situation is this... the plane attempts to move forward but cannot due to the fact that the runway/conveyer is moving the opposite way at the same speed, therefore the plane is standing still and has an airspeed of zero. Of course the plane will not fly! To fly it's need air moving at a certain speed over and under the wings to create lift, therefore, if the plane is essentially standing still and has zero airspeed it cannot lift off.

Bingo. It seems so simple!

[Shiny Lil Detlr]

Originally posted by Jeff Smith
I agree Don, If this were the not the case. Take away the conveyor belt. Lets say we have a parked aircraft, brakes locked, And let's say the brakes were strong enough to hold the bird. Is the aircraft just going to be able to pull enough power to up and fly away? No. The engine is not creating the lift. Only thrust. Gotta have that foreward motion/speed.

That is, unless it's one of those British (or Russian? Can't remember) planes that can hover or take off like a hellicopter due to a rotating engine